From Artefacts to Atoms: The BIPM and the Search for Ultimate Measurement Standards

Terry Quinn wants to redefine the kilogram. This motive drives From Artefacts to Atoms: The BIPM and the Search for Ultimate Measurement Standards, his history of the International Bureau of Weights and Measures (BIPM).1 The title hints at the key question Quinn— BIPM director between 1988 and 2003— raises: what is the significance of transitioning from measurement conventions based on object prototypes to standards tied to fundamental physical constants? The kilogram is the last remaining metrological standard still defined by a physical artifact: the International Prototype Kilogram (IPK), a platinum-iridium cylinder envaulted at the BIPM since 1889. Plans currently underway would replace the IPK with a definition in terms of Planck's constant. Through the history of the BIPM, Quinn, an enthusiastic supporter of these plans, describes an inexorable progression from object standards to absolute standards.

The book's eighteen chapters cleave roughly into thirds. The first six chronicle, in extraordinary detail, the quarter century culminating in the 20 May 1875 signing of the Metre Convention. Quinn describes competing standards in nineteenth century Europe, indicating how the commercial and political forces of an increasingly interconnected world shaped them. The Convention cemented the metric system as the standard for scientific measurement. It also provided for the creation of an institute—the BIPM—to maintain the standard, and a governing committee to oversee the Convention's implementation while navigating the geopolitical squalls that accompanied attempts to forge international consensus around a single system.

The next third recounts, in similar detail, the first half-century of the BIPM's operation. Quinn gives special attention to the creation of the prototype meters and kilograms, the linchpins of the metric system. The task of manufacturing prototypes reliable enough to sustain international confidence tested the mettle of Bureau scientists. The meter, for example, required identifying an alloy with an appropriately low coefficient of thermal expansion, casting that alloy with adequate consistency, and developing exquisitely accurate procedures for testing new prototypes against preexisting standards, among other challenges. Quinn also describes how the BIPM's role expanded in the early twentieth century. The Bureau's mission, originally restricted to maintaining the metric standards, was broadened to encompass all of metrology in 1921. The BIPM had become the world's premier site for high-precision instrument calibration, a status it attained on the power of experimental acumen honed making ever-finer measurements of length and weight standards. The Bureau's position as the source of international confidence in the accuracy of scientific measurement made it the natural institution to do for electrical, thermal, and other quantities what it had already accomplished for length and weight.

The BIPM's expanded purview paved the way for the International System of Units (SI). The push for a standard system of physical quantities begins the final third of the book, in which Quinn describes the transition from artifactual to physical quantities as measurement standards. In 1960 the SI was formally adopted and the first artifact standard became obsolete when the meter was redefined as a multiple of the wavelength of light emitted by krypton 86 during the transition between its 2p10 and 5d5 orbitals. This landmark, for Quinn, was the first step in a process that will likely culminate in a few years with the adoption of the "new" SI and the reclassification of the IPK as a historical object. Quinn closes with a clear synopsis of what is at stake in the debate over whether—or, more realistically, when—to redefine the kilogram. Does the aesthetic allure of a crisp theoretical system outweigh the practical difficulty of measuring new standards to the same accuracy as otherwise antiquated artifacts?

As it weaves its way through the 137 years since the Metre Convention, From Artefacts to Atoms is alternately a disciplinary history of metrology, an institutional history of the BIPM, and a socio-political history of measurement conventions. Readers might find the abrupt transitions between these threads disorienting, compounding the problem that an overabundance of minutia often obscures the larger narrative arc. The net result is twofold. First, the book's primary argument for redefining the kilogram does not emerge as cleanly from the historical exposition as the author might like. Second, these parallel stories only hint at the range of fascinating questions metrology poses, each one of which might sustain a book-length narrative. How does metrology interface with other scientific disciplines? How do the details of laboratory practice ground confidence in measurements? How does the social contract on which measurement conventions are built change when supported by fundamental physical constants rather than objects? This book, ultimately, does not address these questions head on. By suggesting them, however, it indicates the notable role metrology and its flagship institution played in the course of modern science.

1. BIPM is the French initialism of Bureau International des Poids et Mesures.

Joseph D. Martin is a Ph.D. Candidate in the University of Minnesota's Program in the History of Science, Technology, and Medicine and an Interdisciplinary Doctoral Fellow at the Minnesota Center for Philosophy of Science.

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